JP5352808B2 - Hollow calcium carbonate particles synthesized in a non-equilibrium state and method for synthesizing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide calcium carbonate particles having a hollow structure, and to provide a synthetic method for the particles which is improved in productivity and the yield of the calcium carbonate particles and uses synthetic operation conditions being simple and friendly to the environment. <P>SOLUTION: The calcium carbonate particles having a hollow structure are synthesized under a carbonate ionic non-equilibrium condition which is formed by introducing a carbon dioxide gas into an aqueous solution of a calcium salt such as calcium chloride at a relatively high flow rate. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to hollow calcium carbonate particles synthesized in a non-equilibrium state and a synthesis method thereof.

In recent years, particles having a hollow structure (that is, a shell made of a substance and an internal cavity) have been widely used in various fields such as building materials, packaging materials, and clothing. As for the method for producing the hollow particles, for example, Non-Patent Document 1 describes a chemical method using an interfacial reaction, a physicochemical method such as a powder bed method, a mechanical and physical method such as a spray drying method, and the like. Classified and organized. Various methods for synthesizing hollow calcium carbonate particles have been proposed and used.

  For example, in Patent Document 1 (Japanese Patent Publication No. 4-51488), after dissolving calcium chloride or calcium nitrate in an alcohol that is insoluble or hardly soluble in water, an aqueous carbonate solution is added dropwise to the alcohol and mixed with the alcohol. A method has been proposed in which calcium carbonate is generated at the interface between water and water to form hollow particles.

  In Patent Document 2 (Japanese Patent Laid-Open No. 5-154374), the surface of oil droplets in water is coated with calcium carbonate by adding calcium carbonate particles and kerosene (oil) to a calcium chloride solution and stirring. Add glycerin (thickener) and calcium chloride solution to the mixture, gradually add sodium hydroxide solution (pH adjuster) while stirring, and then react with carbon dioxide in the air to coat the oil droplet surface. New calcium carbonate particle microcrystals are deposited between the previously formed calcium carbonate particles to form a shell of calcium carbonate. It is said that hollow calcium carbonate particles can be obtained by immersing this product in ethanol and removing the internal kerosene oil.

  Furthermore, Patent Document 3 (Japanese Patent Laid-Open No. 6-2127938) discloses that a calcium salt aqueous solution is sprayed into a reaction vessel heated to 300 to 1500 ° C. together with carbon dioxide gas, and the calcium salt and carbon dioxide gas are reacted. A method for producing hollow and spherical calcium carbonate particles is disclosed. Further, in Patent Document 4 (Japanese Patent Laid-Open No. 8-169982), by adding carbon dioxide to a mixture of latex and calcium hydroxide, latex particles whose surfaces are coated with calcium carbonate are generated, There has been proposed a method for producing hollow calcium carbonate particles by burning and removing latex core particles by firing.

However, these conventional methods include a process of removing the core material and a process requiring high temperature, and require complicated processes. In view of this, some of the inventors of the present invention (Takahashi, Fuji, Tomioka) et al. In Patent Document 5 (Japanese Patent Laid-Open No. 2005-281034) disclosed a carbon dioxide fine gas of 100 μm or less in an aqueous calcium chloride solution containing ammonia. A method for producing calcium carbonate hollow particles by blowing bubbles and generating and coagulating growth of calcium chloride microcrystals on the surface of carbon dioxide bubbles was proposed.

Japan Powder Industrial Technology Association, "Granulation Handbook", Ohm, March 1991 Japanese Patent Publication No. 4-51488 Japanese Patent Laid-Open No. 5-154374 Japanese Patent Application Laid-Open No.6-127938 Japanese Patent Laid-Open No. 8-169982 JP 2005-281034

  However, the method for producing the calcium carbonate hollow particles proposed in Patent Document 5 is a production method that realizes simplification of the process and low environmental load by using bubbles as a core. There are many unclear points in the formation mechanism, and the yield of forming hollow particles is not satisfactory.

The present invention has been made in view of such a situation, and the problem to be solved is as follows.
An object of the present invention is to provide a calcium carbonate particle having a hollow structure that can be advantageously used as a new material. Further, the calcium carbonate particle having such a hollow structure can be produced in a simple and low-environmental synthetic operating condition with a low productivity. The object is to provide a synthesis method with improved yield.

According to the first aspect of the present invention, there is provided a calcium carbonate having a hollow structure and a method for synthesizing the same, which are formed by directly introducing carbon dioxide into a calcium salt aqueous solution such as calcium chloride at a relatively high flow rate. It is synthesized under equilibrium conditions.
The calcium carbonate having a hollow structure of the invention described in claim 2 and the synthesis method thereof are selected from the solution, pH, temperature, carbon dioxide gas flow rate, etc. in the calcium carbonate having the hollow structure of the invention of claim 1 and the synthesis method thereof. The size and / or form of the calcium carbonate particles having a hollow structure can be controlled by appropriately setting and controlling at least one kind of operation condition.
By controlling the non-equilibrium state by changing the operating conditions such as carbon dioxide flow rate and aeration time, raw material concentration, reaction temperature, pH, etc., the shape and shape of the synthesized calcium carbonate particles are controlled. Under a relatively high flow rate of carbon dioxide gas and high initial pH conditions, a high proportion of hollow calcium carbonate particles can be synthesized,
The invention according to claim 3, that is, a calcium carbonate having a hollow structure having a ring shape, a substantially spherical shape and a spherical shape, having a shell composed of nano-sized vaterite microcrystals having a particle size of about 1 to 10 μm. Can be obtained.

Thus, in the calcium carbonate having a hollow structure of the present invention, by omitting the core particle removal step and the high-temperature operation process, compared to the conventional calcium carbonate particle synthesis method (Patent Documents 1 to 4, etc.), A simple and low environmental impact synthesis process can be realized.
Compared with the method of Patent Document 5, it becomes possible to synthesize hollow calcium carbonate particles without using a special device for generating fine carbon dioxide bubbles and the operation related thereto, thereby further simplifying the synthesis process. Is possible. Thereby, the productivity and yield of hollow particle synthesis can be improved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below with reference to the drawings.

As the raw material solution, a 0.1 mol / L calcium chloride aqueous solution whose pH is adjusted to the alkali side by appropriately adding aqueous ammonia and hydrogen chloride aqueous solution is used. This raw material solution is put into a suitable container, and the temperature is kept constant in the range of normal temperature by immersing it in a thermostat.
Then, carbon dioxide gas is introduced from the cylinder through the flowmeter at a constant flow rate into the appropriately stirred raw material solution. At this time, the carbon dioxide gas does not need to be made into fine bubbles using a bubble generation device or the like, and is directly blown into the solution from tubes of various materials. The introduction flow rate of carbon dioxide gas should be different depending on the volume of the raw material solution. For example, for a 1 L raw material solution, the flow rate of carbon dioxide is about 3 L / min, which is a relatively high flow rate.
If the introduction of carbon dioxide gas is continued for a while, the reaction solution will eventually become clouded visually. This means that calcium carbonate particles were generated. At this time, a series of chemical reactions that can occur in the solution is shown below.

Equation 1 is an equation for the dissolution equilibrium of carbon dioxide gas.

    Equation 2 is a carbonate ion equilibrium equation.

Equation 3 is an equation for the calcium carbonate production reaction.
Part of the carbon dioxide gas blown into the raw material solution is dissolved (formula 1), which reacts with water to become H ₂ CO ₃ , and this dissociation causes the carbonate ion equilibrium as shown in (formula 2) to be on the right side. As a result, the concentration of carbonate ions (CO ₃ ²⁻ ) increases. Calcium carbonate is generated by the reaction of this carbonate ion and calcium ion (dissociated from calcium chloride) (Formula 3). At this time, due to the balance between the effect of increasing carbonate ions by the dissolution of carbon dioxide in the reaction (formula 1) and the effect of consumption of carbonate ions by the formation of calcium carbonate in the reaction (formula 3), this system as a whole is a kind of chemistry. It becomes a non-equilibrium state.
As proof of this, the transmitted light intensity (turbidity) at a wavelength of 850 nm of the reaction solution was measured over time by circulating the reaction solution through the optical flow cell. When the temperature of the reaction solution was 17 ° C., a rapid decrease in transmitted light intensity, that is, formation of calcium carbonate was observed in 430 seconds from the start of carbon dioxide introduction. On the other hand, when the reaction temperature was 28 ° C., the formation of calcium carbonate was observed in 110 seconds.
This fact contributes only to the increase in carbonate ions due to the carbon dioxide gas introduced by shifting the dissolution process of the reaction (Formula 1) and the carbonate ion system equilibrium of the accompanying reaction (Formula 2) to the right. It means that the calcium carbonate formation reaction of the reaction (formula 3) proceeds only when the carbonate ion concentration is sufficiently increased. In addition, the fact that the carbon dioxide gas aeration time required for the start of calcium carbonate production differs depending on the temperature of the reaction solution is due to the temperature dependence of the equilibrium constants of the various reactions shown in the reactions (Equation 1) to (Equation 3). This can be a proof that the entire system is in a non-equilibrium state of a series of equilibrium reactions shown in (Equation 1) to (Equation 3).
Now, the size, form, crystal phase, etc. of the calcium carbonate particles synthesized in this non-equilibrium state vary depending on various experimental operating conditions.
For example, the calcium carbonate produced by aeration of carbon dioxide gas at a low flow rate of 2 mL / min for 2 hours with respect to 250 mL of the raw calcium chloride aqueous solution has a large number of scale-shaped particles. On the other hand, for example, when a high flow rate such as a carbon dioxide flow rate of 3 L / min is used with respect to a 1 L raw material solution, white turbidity of the solution was observed in a few minutes under the condition of a reaction temperature of 28 ° C. and obtained. Many hollow particles were observed in the calcium carbonate particles.
Several examples of SEM (scanning electron microscope) images of the synthesized calcium carbonate particles under these conditions are shown in FIGS. These particles were obtained by aeration of carbon dioxide gas until the initial pH of the raw material calcium chloride solution was 10.30 (Fig. 1), 10.00 (Fig. 2), and 9.55 (Fig. 3), and this was reduced by 0.15. is there. From these figures, it can be seen that hollow calcium carbonate particles having a particle diameter of approximately 1 μm are formed. Here, the fact that the internal voids can be seen in the SEM image is considered to be a result of fracture of the synthesized hollow particles in any step of the experimental operation, or a state of the formation process of the hollow structure. 1 and 2 show many tubular hollow particles, and FIG. 3 shows many substantially hollow and spherical hollow particles. This indicates that it is possible to synthesize hollow calcium carbonate particles having various forms by controlling the pH in a non-equilibrium state.
The calcium carbonate particles obtained when these carbon dioxide gases were aerated at a high flow rate were in the vaterite crystal phase, whereas some calcite crystal phases were also observed when the aforesaid low flow rate aeration was performed. It was. This indicates the possibility of controlling the crystal phase of the calcium carbonate particles obtained by controlling the non-equilibrium state.
As described above, the present invention changes the experimental operating conditions (carbon dioxide flow rate and aeration time, raw material concentration, reaction temperature, pH, etc.) in the chemical non-equilibrium state consisting of the reactions (Formula 1) to (Formula 3). The shape and shape of the synthesized calcium carbonate particles can be controlled by controlling the amount of hollow calcium carbonate particles at a high rate, especially under a relatively high flow rate of carbon dioxide gas and high initial pH conditions. It is essential to be able to do this.
In the above embodiment, a water-soluble salt other than calcium chloride can be used as a raw material. For example, calcium hydroxide, calcium nitrate, calcium bromide and the like can be used.
In addition, by appropriately setting and controlling operating conditions such as solution pH, temperature, carbon dioxide gas flow rate, etc., the size and form of the hollow calcium carbonate particles to be synthesized can be controlled. By changing the calcium concentration, the pH can be adjusted to, for example, pH 8 to 13, preferably 10.30 to 9.40, and the reaction temperature can be performed at a constant temperature in the range of 17 ° C to 28 ° C. The introduction flow rate of carbon dioxide gas is 3 L / min gas flow rate for 1 L of reaction solution, or for example, 0.1 L / min to 10 L / min gas flow rate. Further, a combination of a gas flow rate and a reaction solution amount similar to this ratio can be used. Various materials (metal, plastic, etc.) can be used as the carbon dioxide gas introducing tube, and the diameter thereof is, for example, several millimeters to several centimeters.

  The calcium carbonate having a hollow structure of the present invention and the synthesis method thereof are expected to be used as a novel material such as a molecular filter because the calcium carbonate having a hollow structure has characteristics such as high porosity and bulk density. There is industrial applicability as a method for synthesizing hollow calcium carbonate particles easily and inexpensively.

FIG. 2 is an SEM (scanning electron microscope) image of synthesized calcium carbonate particles when the reaction temperature is 28 ° C., the initial pH of the raw material calcium chloride solution is 10.30, and the pH at the end of the synthesis experiment is 10.05. It is a SEM (scanning electron microscope) image of the synthesized calcium carbonate particles when the reaction temperature is 28 ° C., the initial pH of the raw material calcium chloride solution is 10.00, and the pH at the end of the synthesis experiment is 9.85. It is a SEM (scanning electron microscope) image of the synthesized calcium carbonate particles when the reaction temperature is 28 ° C., the initial pH of the raw material calcium chloride solution is 9.55, and the pH at the end of the synthesis experiment is 9.40.

Claims (2)

pH 8 to 13, the calcium chloride aqueous solution temperature of 17 ° C. ~ 28 ° C., without the carbon dioxide gas into a fine bubble, at a flow rate of 0.1L / min~10L / min to the aqueous solution 1L A method for synthesizing calcium carbonate having a hollow structure, which is synthesized under carbonate ion non-equilibrium conditions formed by direct introduction. 2. The method for synthesizing calcium carbonate having a hollow structure according to claim 1, wherein the particle size is 1 to 10 μm and the shape is a tube shape, a substantially spherical shape or a spherical shape having a shell composed of nano-sized vaterite microcrystals.